Tag Archives: galaxy

Post navigation

Editor’s note: This blog is one of a series of travelogues by HMNS VP of Astronomy Carolyn Sumners, sent from the two-week solar eclipse viewing trip she led to the South Pacific. Astrophotography by Gary Young.

The stars of the southern hemisphere are fantastic, with the brilliant Milky Way stretching from near the hunter, Orion, in the North to Crux, the Southern Cross, in the South.

This predawn image is a time exposure with a Canon Mark II camera and a fisheye lens, taken from our hotel lawn in Queenstown, New Zealand, looking out over Lake Wakatipu. Even with some glare from the hotel and from Queenstown to the East, the predawn sky is remarkably dark. This exposure approximates what we could see as we faced south.

New Zealand has a total population less than 5 million, which guarantees much less light pollution, even close to a city. Also, the southern Milky Way is much richer and more easily seen than the Milky Way near the North Star. In New Zealand, we trade views of the Big and Little Bears for the Southern Cross and the nebulae around it. This image is a close-up of the southern Milky Way in the early evening as we started stargazing. Notice the dark areas in the Milky Way. The Inca saw animals in these dark dust clouds.

While observing through two telescopes, we placed our third telescope, a Takahashi FS60Q, on a small portable Sky Patrol equatorial mount that would track the stars — adjusting for the Earth’s rotation. We were able to do time exposures of up to a minute without guidance and we captured incredible views of the Orion Nebula, the Omega Centauri globular cluster, the Large and Small Magellanic Clouds and the Carina Nebula.

Large and Small Magellanic Clouds

Through a telescope we saw the shapes of these clouds and clusters, but not the rich colors and textures captured in these images. The Orion Nebula is a stellar birth cloud with new stars still forming from the gas and dust. The Carina region has young stars and the dying supergiant Eta Carinae.

Eta Carinae nebula

Omega Centauri is the brightest globular cluster in Earth skies with 5 million stars. The Large and Small Magellanic Clouds are satellite galaxies of our Milky Way galaxy. We can see the Orion Nebula easily from Houston. The other magnificent objects are best seen from below Earth’s equator.

As you look up into a November sky right at nightfall, you may notice fewer bright stars than at other times of year. No, it’s not just the glare from Houston hiding most of the stars from view–there really are fewer bright stars in the November evening sky than in, say, February or August. To understand why, you need to understand the shape of our galaxy itself.

Evidence indicates that the Milky Way, like many large galaxies, has a massive black hole at its center. A radio source designated Sagittarius A* could be the black hole itself. (The asterisk is part of the name, which is “Sagittarius-A-star”). Surrounding this black hole is a central bulge where older (and thus redder) stars predominate. The Bulge of our galaxy is not fully spherical but instead forms a bar a few thousand light years long. Branching out from this bulge are spiral arms which contain younger (bluer) stars and dust clouds out of which brand new stars form. Our solar system is about 26,000 light-years from the center to the edge, on the inside edge of the Orion Arm. The Orion Arm, in turn, is but a spur of the much longer Perseus Arm. The Milky Way is quite flat–over 100,000 light years wide but only 1,000 light years thick.

The flatness of the galaxy means that most of its stars are near a certain plane in space. Of course, the galaxy is much thicker than our solar system, so we see our stellar neighbors suurounding us on all sides. The rest of the galaxy, extending off into the distance, appears to us as a hazy blur in the background, with individual stars (those fairly close to us) in the foreground. That hazy blur looked like spilled milk to the ancient Greeks, thus the name ‘Milky Way.’ We see more stars near that plane than far from it.

What does this have to do with the dimness of a late November sky at dusk?

Imagine observing our flat galaxy from our vantage point on Earth. When we face into the galactic plane, we see more bright stars, because there are more stars in that direction. When we face above or below that plane, we see fewer bright stars.

Face west at dusk in late November and early December, and you’ll notice an enormous triangle of three bright stars, all bright enough to appear even in skies lit by Houston. These stars from the Summer Triangle, so called because it is up all night long from June through mid-August. This Triangle is also directly in the plane of the Milky Way. The constellation Sagittarius, which marks the center of the Galaxy, sets just after the Sun this time of year. Therefore, if you trace a path approximately from the point of sunset through the Summer Triangle, over to five stars in an ‘m’ shape in the North (that would be Cassiopeia, the Queen), and then over to the northeastern horizon. This is the plane of the Milky Way across late autumn skies at dusk .

Turn to the south, and you face below the galactic plane (as we’ve arbitrarily defined ‘above’ and ‘below’). Here is a vast region of sky almost void of bright stars. One exception is Fomalhaut, low in the southeast at dusk tonight. Also, Houstonians with a very clear southern horizon can see Achernar very, very low in the south on December evenings. But that’s about it. There are many fewer bright stars in this direction than towards the Summer Triangle. By the way, the brilliant object in the east at dusk tonight, and high in the southeast as dusk in December, is Jupiter. It doesn’t count as a bright star for this sector of the sky.

The Celestial Sea

When ancient Mesopotamians looked up into the dim skies you see at dusk tonight, they imagined the Persian Gulf south of them extended up into the sky, forming a ‘Celestial Sea’. They therefore placed many water-themed constellations in this part of the sky. Zodiacal constellations here include Pisces, the Fish, and Aquarius, the Water-Carrier. Even Capricornus, the Goat, has the tail of a fish because he originally represented Ea, the ancient Babylonian god of the waters. Under Pisces is the sea monster Cetus, while Piscis Austrinus, the Southern Fish, drinks the water that Aquarius pours. Eridanus, the River, rises in the southeast, flowing from Orion’s foot into this vast ‘sea.’

Contrast this vast, dim region with the much brighter swath of stars that rises in the east later this evening (9-10 pm in late November, earlier in December). This region of sky includes the brilliant pattern Orion, the Hunter, as well as Sirius, the brightest star we ever see at night. When these stars rise, we are beginning to face back into the plane of our galaxy, this time looking into our own arm of galaxy at the stars right ‘behind’ the Sun. (This is why our arm of the Milky Way is called the Orion Arm.) Winter evening skies are much brighter than those of late autumn.

Saturn is now in the south southeast at dusk. We are seeing its rings a little more edge on than earlier in the year. In fact, Saturn’s rings won’t be this edge-on to us for another 15 years. Saturn, like Earth, is tilted on its axis (at 26.7 degrees, Saturn’s tilt is a little greater than Earth’s). Twice per Saturn orbit, then, about every 15 years, Saturn has equinoxes where the sun is aligned with Saturn’s equator. Since the rings orbit the equator, this puts the sun (and the Earth) in Saturn’s ring plane. Earth was exactly in Saturn’s ring plane on September 3, 2009 when Saturn was also on the far side of the sun and hard for us to see. This month, Earth again approaches (but will not cross) Saturn’s ring plane. That’s why the rings appear so thin in telescopes now. Learn more about the rings of Saturn in my latest blog post.

Venus keeps getting higher in the evening sky during May. Face west at dusk and look for a point of light that outshines everything in the sky but the sun and the moon.

Mars is very high in the evening sky, although not as bright as it was in winter. Since January 29, Earth has been pulling ahead of Mars on its faster orbit. As a result, Mars gets slightly dimmer each night for the rest of 2010. However, during May, Mars remains brighter than average, and thus remains easy to see. Look high in the west at dusk for a reddish point of light.

Jupiter is low in the southeast at dawn this month. Look for it low in twilight as day begins to break. It will be higher in the southeast by the end of the month.

In May, you can watch as the Dog Days begin! We are in the Dog Days when the Dogs have vanished from the sky. As May begins, Orion, the Hunter is clearly visible due west right after sunset. To his left, aligned with Orion’s belt, is Sirius, the Dog Star, the brightest star we see at night. Forming a triangle with Sirius and Orion’s brightest star Betelgeuse is Procyon, the Little Dog Star. Throughout May, watch as Sirius appears slightly lower and lower to the horizon each night, until it is gone by May 31. By mid-June, Procyon is gone as well. When the Dogs are up only in the day, we’re in the Dog Days.

Meanwhile, spring stars are high in the south and east. A distinct backwards question mark shape outlines the mane and forepaws of Leo, the Lion. Three stars forming a right triangle rise underneath; they mark Leo’s hindquarters. The Big Dipper is as high as it ever gets in the north at dusk. You can extend the curve of its handle to ‘arc to Arcturus’ and then ‘speed on to Spica’. These stars high in the east and southeast, respectively, by dusk tonight.

As Orion and Sirius set, the plane of the Milky Way largely coincides with the horizon. (At Houston’s latitude, the two planes are off by less than three degrees). We are therefore looking straight out of the Milky Way plane when we look up early on a May evening. Thus May evenings have fewer bright stars, as most of the brightest stars in the Milky Way plane are ringing the horizon.

Saturn, up all night long last month, can now be found in the east southeast at dusk. We are seeing its rings a little more edge on than earlier in the year, an effect that gets even more pronounced next month.

Venus keeps getting higher in the evening sky during the month of April. Face west at dusk and look for a point of light that outshines everything in the sky. As April opens, Venus has a companion; the elusive Mercury is to its lower right. Normally too close to the sun to appear in our night sky, Mercury has come from behind the sun and appears far enough to its side that we can still see it just after sunset. Mercury’s greatest elongation (apparent distance from the sun) occurs on April 8. After that date, we see Mercury return towards the sun’s glare.

Mars is very high in the evening sky, although not as bright as it was in winter. Since Jan. 29, the Earth has been pulling ahead of Mars on its faster orbit. As a result, Mars gets slightly dimmer each night for the rest of 2010. However, during April, Mars remains brighter than average and thus remains easy to see. Look high in the south at dusk for a reddish point of light.

Jupiter is low in the southeast at dawn this month. Look for it low in twilight as day begins to break. It will be higher in the southeast by the end of the month.

Johannes Hevelius drew the Orion constellation
in Uranographia, his celestial catalogue in 1690

Now that the winter is over, the winter stars have shifted to the west. Dazzling Orion is high in the southwest. His belt points right to Aldebaran, the eye of Taurus, the Bull. The Dog Stars Sirius and Procyon are to Orion’s left. Sirius is the brightest star we ever see at night. Gemini, the Twins, are to Orion’s upper left. Look for two stars of equal brightness less than 5 degrees (three fingers at arms’ length) apart. These are Castor and Pollux, marking the twins’ heads. High in the northwest is Capella, the sixth brightest star ever seen at night.

Meanwhile, the spring stars are high in the east. A distinct backwards question mark shape outlines the mane and forepaws of Leo, the Lion. Three stars forming a right triangle rise underneath; they mark Leo’s hindquarters. The Big Dipper is high in the northeast at dusk. If you have a clear eastern horizon, you can extend the curve of its handle to ‘arc to Arcturus’ and then ‘speed on to Spica.’ These stars are along the eastern horizon by dusk tonight.

The large contrast between the bright winter stars in the west at nightfall and the dimmer spring stars in the east arises because of the shape of our Milky Way. The Galaxy is a barred spiral much thinner than it is wide across. Thus, most stars are near the plane of the galaxy. Orion, Taurus, Gemini, and the Dogs are near the galactic plane, while Arcturus and the stars of Leo and Virgo are far above it.